The present invention relates to an addressing method and apparatus for liquid crystal devices having particular, but not exclusive, application to large area ferroelectric liquid crystal devices.
Liquid crystal display and shutter devices have been known for many years for use in calculators, computer displays and so on. One well known type of display uses a twisted nematic liquid crystal in which liquid crystal molecules are induced by a pair of substrates to form a spiral having an axis perpendicular to the pair of substrates which contain the liquid crystal. Electrodes arc arranged on the substrates and applying a voltage, via these electrodes, across the liquid crystal causes the liquid crystal molecules to xe2x80x9cunwindxe2x80x9d. Since the molecules are optically active the polarisation of any light passing between the substrates is altered when the molecules form a spiral. By using a polariser on the substrates of the device, light and dark states may be provided dependent upon whether the voltage is applied or not.
Two major disadvantages with twisted nematic liquid crystal devices are that the speed of response is low and that the molecules relax (albeit quite slowly) to a single stable position when the drive voltage is removed. In a large area display device, some form of multiplexing is required to address the large number of liquid crystal pixels because individual electrical connections to drive each pixel are not possible. Since the molecules relax back to their stable state in the absence of a drive voltage, they must be addressed at frequent intervals if the contrast of the display is to be adequate. The rate at which the whole array can be addressed is therefore limited. In addition, the response speed of nematic liquid crystal displays makes them generally unsuitable for displaying moving images (i.e. video rate).
A faster response may be provided by liquid crystal display and shutter devices based upon ferroelectric liquid crystal materials. These are generally very thin devices in which the spontaneous helix of the chiral smectic phase is retained in an xe2x80x9cunwoundxe2x80x9d state by the physical restriction of closely-spaced substrates. By selection of liquid crystal material, the state of the material and physical dimensions, a device which exhibits bi-stability may be provided. When a voltage of a first polarity is supplied across such a liquid crystal device the liquid crystal molecules adopt a first position or state provided that the time and voltage product (xcfx84V) is above a certain threshold. When a voltage of the opposite polarity is applied (and also above the xcfx84V threshold), the molecules adopt a second position or state. The molecules are optically active so, using polarisers, these two positions can give different states of optical transmission. The molecules, or more properly the director, will remain in the adopted positions until a voltage of the opposite polarity is applied. Such devices, as well as exhibiting bi-stability, also offer a faster response than twisted nematic devices. Both of these characteristics makes liquid crystal devices based on ferroelectric liquid crystals attractive for use in large area liquid crystal array devices (be they displays, shutters and so on).
Typically, electrodes will be arranged on the two substrates of the device in respective orthogonal directions to form a matrix of picture elements called pixels. The points at which the electrodes on a first substrate (row electrodes) cross those on a second substrate (column electrodes) define the pixel elements of the array. Some form of multiplexing must be used to address such a device. A common technique is to apply a blanking signal followed by a strobe signal to all of the rows of the device in succession. The blanking signal is arranged to ensure that all of the pixels in a row adopt the same state (typically dark) irrespective of the data signal applied to the columns. Then, a strobe signal is applied to a row electrode while the states of the pixels in that row are altered as appropriate using a number of data signals applied to the column electrodes. One of two data signals is usually provided to each column electrode, a SELECT data signal which causes the addressed pixel to change state and a NON_SELECT data signal that causes the pixel to remain in the same state in other words the state induced by the blanking signal. The strobe signal is applied to each row for a long enough period of time to permit the ferroelectric liquid crystal material at each pixel to adopt the desired state and then the strobe signal is applied to the subsequent row. The data signals applied to the columns of the device are then altered to correspond with the desired states of the pixels in that subsequent row. The time required, therefore, to address the whole array is equal to the strobe signal application time (per row) multiplied by the number of rows in the array. Further details can be found in UK patent publication number GB 2,232,802.
The term xe2x80x9cstrobe signalxe2x80x9d is issued to refer to that portion of a signal applied to a first electrode of a device which, in co-operation with a particular signal applied to a second electrode, provides discrimination between the states adopted by an addressed pixel. The strobe signal may include portions before and after the duration of the particular signal applied to the second electrode.
As the size and resolution of the display or shutter devices increases, the number of rows increases and/or the frame rate (for example that set by a requirement for no-flicker) may be such that the ferroelectric liquid crystal material has insufficient speed to be addressed whilst maintaining discrimination between the bistable states across the device.
A partial solution to this problem, is to address the array in two halves. Two sets of column electrodes are provided which extend from opposite edges of one of the substrates. Two strobe signals are applied at any given time, one strobe signal to one half of the array and the other strobe signal to the other half of the array. Because two rows of the array can be addressed simultaneously the frame rate is effectively doubled. The main drawback of such an arrangement is that the addressing circuitry for providing the data (column) signals has to be duplicated. Providing interrupted column electrodes and the extra connections to the addressing circuitry also increases the expense of the arrangement.
Another problem is temperature. A liquid crystal material may operate sufficiently quickly at some temperatures but if the temperature of the device is lower, the material is slower and the frame rate cannot be maintained. This may be cured to some extent by extending the strobe signal into the following lines still further (as in United Kingdom Patent Publication GB 2,262,831) but this reduces the temporal/voltage operating range for some data patterns in the following lines. Thus, there is a limit to which the strobe may be extended and hence temperature of operation compensated for.
For a large area display there may be significant variations of temperature, alignment quality, pixel pattern switching history and/or voltage (signal shape and magnitude, for example caused by line transmission problems). Another problem is limited operating range (for example in time/voltage) over which discrimination between the select and non_select resultant signals deteriorates. This can mean that operation does not occur uniformly across the whole panel and the two bistable states may not be obtained reliably.
It is an object of the present invention to provide an addressing method and apparatus for a liquid crystal device which ameliorate these disadvantages.
According to the first aspect of the present invention, there is provided a method of addressing a liquid crystal device comprising applying a first signal to one of a first plurality of electrodes of the device and applying a second signal to one of a second plurality of electrodes of the device which first and second signals address a pixel of the device, the second signal comprising one of at least a first data signal and a second data signal which first and second data signals are different from each other and each have a duration and a discriminating duration, which is no longer than the duration, during which discriminating duration the first data signal differs from the second data signal, wherein the first signal comprises a first portion applied during the discriminating duration of the second signal and a second portion applied after the discriminating duration of the second signal which second portion of the first signal includes a section at a larger voltage magnitude than the first portion of the first signal.
It has been appreciated that ferroelectric (and also antiferroelectric or AFL) liquid crystal devices may be addressed by a combination of strobe and data signals in which only an earlier portion of the combined signal differs for SELECT and NON_SELECT of the pixel allowing the next line or lines to be addressed before complete latching of the pixels of the first line. Prior attempts to use such techniques have resulted in poor discrimination between the combined signals for the SELECT and NON_SELECT cases, in other words a reduction in confidence that the state adopted by a pixel is the state requested by the applied data signal. This is commonly the result of changes in temperature, applied voltage, alignment of the liquid crystal and so on.
It has now been appreciated that improvements in one or more parameters (such as an increase in the addressing speed ) can be obtained without such a serious reduction in the discrimination. This is achieved by increasing the voltage of the strobe signal (first signal) applied to a row of the device once (or some time after) the discriminating SELECT or NON_SELECT part of the data signal (second signal) for that row is no longer applied to the column electrodes. This can be used to enhance the frame rate by reducing the amount of time that each row needs to be addressed with its respective data signals. Alternatively or in addition, the problem of poor discrimination between SELECT and NON_SELECT states of pixels is reduced by the section of the strobe signal having a larger voltage magnitude for reasons discussed more fully below.
The present invention also relates to a corresponding addressing arrangement and a liquid crystal device comprising such an arrangement.
According to a second aspect of the present invention, there is provided an apparatus for addressing a liquid crystal device comprising means for applying a first signal to one of a first plurality of electrodes of the device and means for applying a second signal to one of a second plurality of electrodes of the device which first and second signals address a pixel of the device, the second signal comprising one of at least a first data signal and a second data signal which first and second data signals are different from each other and each have a duration and a discriminating duration which is no longer than the duration, during which discriminating duration the first data signal differs from the second data signal, wherein the means for applying the first signal is arranged to provide a first signal comprising a first portion applied during the discriminating duration of the second signal and a second portion applied after the discriminating duration of the second signal which second portion of the first signal includes a section at a larger voltage magnitude than the first portion of the first signal.
According to a third aspect of the present invention there is provided a liquid crystal device comprising an array of electrodes arranged on substrates for containing a liquid crystal, means for applying a first signal to one of a first plurality of electrodes of the device and means for applying a second signal to one of a second plurality of electrodes of the device which first and second signals address a pixel of the device, the second signal comprising one of at least a first data signal and a second data signal which first and second data signals are different from each other and each have a duration and a discriminating duration which is no longer than the duration, during which discriminating duration the first data signal differs from the second data signal, wherein the means for applying the first signal is arranged to provide a first signal comprising a first portion applied during the discriminating duration of the second signal and a second portion applied after the discriminating duration of the second signal which second portion of the first signal includes a section at a larger voltage magnitude than the first portion of the first signal.
Usually the liquid crystal will be a liquid crystal mixture comprising at least one liquid crystal-forming compound.
In an embodiment of the invention, a strobe signal will be applied simultaneously to at least two consecutive rows of the device. During the second (or xe2x80x9cextendedxe2x80x9d) portion of the strobe signal, the pixels of the row to which it is applied will also be subjected to the data signals intended for the pixels of the following row. Clearly, these signals will not necessarily be the correct ones for the pixels of the row to which the extended portion of the strobe signal is applied. The present invention is based on a study of the performance characteristics of ferroelectric liquid crystal and a realisation that by modifying a second or extended portion of the strobe signal, the effect of the data signals applied to the pixels of that row during that extended portion can be minimised. Thus the line address time of the device which is equal to the frame rate divided by the number of rows can be shortened. The time taken to address the whole array is consequently reduced or the temperature range for a given frame rate can be extended.
In one embodiment of the present invention, the whole of the second portion of the first (strobe) signal may have a higher voltage amplitude than the initial portion.
Part of the second portion of the first (strobe) signal may be altered to provide a temperature compensation for the liquid crystal device. This may entail making the second portion of the strobe signal include a portion at a lower voltage than the highest voltage of the second portion or even the highest voltage of the first portion of the signal. This lower voltage portion can also provide improved discrimination for the device.
The duration of the second portion of a strobe signal may be varied within the scope of the invention. If the second portion is arranged to be no longer than a line address time (L.A.T) then the driving circuitry for the strobe signal needs only to provide a strobe signal to two of the rows of the device simultaneously. However, the second portion of the strobe signal may be lengthened beyond one line address time in order to provide a higher addressing speed, improved discrimination, temperature compensation or any combination of these. Limiting the second portion to one L.A.T. can provide a simplification in the strobe signal driving circuitry.
According to an embodiment of the present invention, the second portion of the first (strobe) signal is arranged to approximate an optimum switching voltage signal for the liquid crystal independent of the data signal applied. Some compromise is necessary here, since the second (data) signal applied to the subsequent row(s) is not constrained.
In a preferred embodiment of the present invention, the line address time of the addressing method or apparatus is arranged to be less than the latching time of the ferroelectric liquid crystal material.
In a further embodiment of the invention, at least a portion of the strobe signal is arranged to be a continuously varying signal, for example the second portion of the strobe signal.
Further preferred features of the present invention are set out in appended dependent claims.